Frequency changer



law. 2, 1940; vy. P. OVERBEcR I FREQUENCY CHANGER Fild Feb. 19, 1938 5 Sheets-Sheet 1 W/Lmx P OVZ'ERBECK Jan. 2, 1940. W. F. OVERBECK FREQUENCY CHANGER 5 Sheets-Sheet Filed Feb. 19, I L958 [mm W/LOOX POvERBEcK Jan. 2, 1940.

Akvaas Var/155500014 5 MAI-CU/Ifll/Lf/DN w. P. OVERBECK 2,185,418

-FREQUENCY CHANGER Filed Feb. 19, 1938 5 SheetsSheet 4 I fln/MAAW 147471455 ml- .W/Lmx P OVERBEC'K Jan. 2, 1940. w.'P. OVERBECK FREQUENCY CHANGER Filed Feb. 19, 1938 5 Sheets-Sheet 5 r w/MARY [HERE/V7 Pam/40y I04 71465 6= mws UEZAV-DEE/PEES WILCO" oyfRgfcK Patented Jan. 2, 1940 2,185,418 FREQUENCY CHANGER Wilcox P. Overbeck, Waltham, Mass., assignor to Baytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application February 19, 1938, Serial No. 191,507

15 Claims. (01. 172-281) This invention relates to the. conversion of power from commercial power lines to electrical currents at higher frequencies, particularly to those frequencies which are simple multiples of the power line frequency. It also relates to the conversion of electrical power .from plural-phase lines to single-phase power. A frequency change is useful for various purposes; for example, the operation of high speed machinery converting power to audible frequencies for signalling purposes and for induction heating. Conversion of power from plural-phase to single-phase is usein] for operating single-phase devices without creating an unbalanced load on the multiphase lines.

In the present application; when frequency conversion is referred to, it is intended that this shall include the conversion of multi-phase to single-phase as well as the multiplication of frequencies.

Prior methods and devices for producing frequency conversion have been objectionable because of the use of moving parts, limitations in size and efficiency, poor regulation withchanges in loador poor power factor of the load drawn from the power lines.

An object of this invention is to produce a device for multiplying power frequencies which contains no moving parts.

Another object is to produce such a frequency multiplier which is reasonably efiicient, has substantially no limitations as to size, and which has good regulation with changes in load.

A further object'is to produce a frequency multiplier which will present a load of good power factor to .the power lines, and which when designed for operation from three-phase lines will present a balanced load to all three phases.

An additional objectis to produce a device for converting multi-phase power to single-phase power which also has the desirable characteristics enumerated above.

A still further object is to devise an arrangement employing gas-filled tubes of the grid-controlled or magnetic-controlled typeto produce the desired results.

The foregoing and other objects of my invention will be'best understood from the following description of exemplifications thereof, reference being had to the accompanying diagrammatic drawings in which a Fig. 1 represents a circuitembodying one form of my invention which will convert a single-phase power of one frequency to single-phase power 01 double frequency;

Fig. 2 is a circuit diagram of another embodiment of my invention for converting three-phase power of one frequency to single-phase power of triple frequency;

Fig. 3 represents a third form of my invention for converting three-phase power of one frequency to three-phase power of triple frequency;

Fig. 4 is an oscillographic diagram showing the relationships between various voltages and cur- 1 primary 2 is adapted to be connected to a suit- 20 able source of single-phase alternating current power. The transformer I has a secondary 3 with a mid-tap 4. The ends of the secondary 3 are connected by means of leads 5 to the anodes 6 of a pair of controlled electrical space discharge tubes 1. The tubes are likewise each provided with a cathode-8 which preferably is of the indirectly-heated thermionic type.

The tubes 1 are each filled with a suitable ionizable gas or vapor at a sufliciently high pressure so that the filling becomes strongly ionized upon the passage of a discharge between each anode 6 and its associated cathode 8, whereupon conduction of current at relatively low voltage drop occurs. This gas might be mercury vapor at suitable pressure, forexample, between 1 and 100 microns, or a noble gas, such as argon, at a pressure of the order of one millimeter or less. In

any event, the gas pressure is of a suitable value so that the ionization and low voltage conduction mentioned above may occur. Gaseous discharge tubes of this type may be adapted to control the initiation of the discharge between each cathode and anode. For example, an electrostatic grid intermediate these electrodes may delay the initiation of the discharge between the cathode and anode after the anode becomes positive until the voltage on the intermediate grid drops to a predetermined minimum value, whereupon initiation of the discharge occurs and conduction continues until the anode no longer is positive with respect to its associated cathode. The control of the initiation of a discharge can likewise be accomplished by magnetic means, and in the present arrangement I prefer to use this latter type of control.

.In order to utilize magnetic control of the starting of the discharge through each of the tubes I, these tubes are preferably constructed as more fully described and claimed in the copending application of Percy L. Spencer, Serial No, 612,235, filed May 19, 1932, for an improvement in Electrical gaseous discharge devices. As pointed out in said application, each of the tubes I is provided with an intermediate electrode 9 surrounding the discharge path between the cathode 8 and anode 6. This intermediate electrode is made of some suitable non-magnetic electrically-conducting material, and may be made in the form of a cylinder open at both ends so that the discharge when initiated may pass freely between each cathode 8 and anode B. Each of the intermediate electrodes 9 is preferably connected by means of a conductor II] to its associated cathode 8. Said intermediate electrodes may be connected to other parts of the circuit and may have'impressed upon them various bias voltages if desired. In such tubes, if a transvese magnetic field is impressed upon the discharge space within the intermediate electrode 9, initiation of the discharge after the anode becomes positive is delayed until the transverse magnetic field falls to a predetermined minimum, which for all practical purposes may be considered as being substantially zero. In order to provide such a transverse magnetic field, each tube I has associated therewith a control coil II wound upon a suitable core I2. The core I2 is provided in any suitable manner with pole pieces which impress a transverse magnetic field across the tube I associated therewith.

The cathodes 8 of the tubes I are intercomnected by means of a conductor I3. A lead I4 extends from the common conductor I3 to one end of the primary winding I 5 of an output transformer I6. The other end of the primary winding I5 is connected by means of a conductor I'I through a current-limiting resistance I8 to the center tap 4 on the secondary winding 3 of the input transformer I. The transformer I6 is provided with a secondary winding I9 which is connected by means of leads "to a suitable load device 2|.

The resistance I8 is provided for the purpose of limiting the direct current which flows in the primary winding I5, and for this purpose may be of relatively low value. As a matter of fact, in some instances the resistance. of the winding I5 in itself may be sufficient to restrict the direct current to a suitable value. Also, due to the fact that direct current flows in the primary winding I5, the transformer I6 is provided with a magnetic core which preferably has an air gap so as to prevent the direct current from saturating the iron of said core.

In order to correct the power factor of the load drawn from the alternating current source, a condenser 22' is connected across the primary .winding 2.

I have found that the circuit as described above can be made to deliver double frequency to the load device 2I, having the desired characteristics as mentioned in the objects of the invention, if the starting of the discharge through each tube 1 is delayed until a time almost 90 degrees later in the alternating current cycle than the time at which the respective anodes 6 should become positive with relation to their respective catho es 3 In order that this delay may take place, each control coil II is energized through a control device 84, which may in turn be supplied with power from the secondary winding 3. For this purpose a pair of conductors may connect the input to the control device 84 to the opposite ends of the secondary winding 3. The control device 84 is so adjusted as to cause the current flowingthrough each control coil I I to be substantially degrees out of phase with the voltage across the secondary winding 3. Under these conditions, the current and likewise the magnetic field through each coil I I passes through zero substantially 90 degrees after each anode 6 becomes positive with respect to its associated cathode. 1 When the circuit is so adjusted, the desired 90 degree delay in the starting of the discharge is obtained.

The behavior in the circuit as described above is illustrated in Fig. 4. Along axis a of Fig. 4 is 2 a wave shown as a solid line which represents the voltage across the primary I5 of the transformer I6. The dotted lines represent the voltage applied to the anodes 6 of the tubes I during the periods when no conduction takes place. 2 The two tubes in Fig. 1 may be represented as tubes A and B. As illustrated along axis a of Fig. 4, two alternating voltages degrees out of phase with each otherare impressed across the tubes A and B. As is well known, if the two tubes were operating as in a normal full-wave rectifying system, each tube would fire or start conducting current at substantially the beginning of the conducting half of the voltage wave impressed upon it, and would stop conducting at 3 substantially the end of said half wave. As pointed out above, in the present system the start of conducting is delayed by a substantial phase angle beyond the normal firing point, and thus, it will be seen that each tube starts to conduct substantially 90 degrees after its anode becomes positive. Also the end of the conducting period is delayed substantially beyond the end of the normal conducting half of the voltage wave, and each tube preferably continues to conduct until 4 the other tube starts at a corresponding time. The voltage across the primary I5 oi the output transformer I6 necessarily follows the anode voltage to whichever tube is conducting, and this voltage is represented by the solid wave along 5 axis a, as pointed out above. It will be noted that this solid wave is double the frequency of the voltage applied to each anode, and therefore double the frequency of the alternating current source. The secondary 19, therefore, delivers this alternating voltage of double frequency to the load device 2I.

One of the requirements of the operation as analyzed above is that each tube shall conduct current until the other tube starts. Whether or not this occurs depends upon the inductance of the circuit consisting of the conductor I I, the primary winding I3, the resistance I8 and the conductor II. In most instances this inductance consists substantially of the inductance of the primary winding I3. The dependence of the maintenance of conduction, through each tube for the desired period of time upon such inductance is more fully described and claimed in my 1 copending application, Serial No. 186,372, filed January 22, .1938, for an improvement in Filter systems for controlled rectiflers, in which the methods whereby the necessary critical inductance may be calculated are set forth. In Fig. I

7 of the drawings is a chart which indicates the value of which is necessary in order that one tube shall continue to conduct until the other tube starts as a function of the angle of delay in the starting of the tubes of Fig. 1. The symbol (0 represents 21? times the frequency of alternating current power line; 110 represents the required critical inductance of the transformer primary l5; and R represents the value of the resistance l8. This resistance may be a small separate resistance as shown or may represent the resistance of the output transformer primary l 5. The upper curve labeled n=2 represents the conditions of Fig. 1. The symbol n=2 indicates that two rectifying phases or two rectifying paths are present in the arrangement. As will be seen from Fig. 7, as the firing delay of the tubes in Fig. 1 approaches degrees, the value of increases very rapidly. However, in the case where the resistance l8 represents the resistance of the primary winding 15, the value of is generally of the order of 50, although with the use of special core materials, it is possible to obtain higher values. In Fig. 7, values of (.cL R

only up to 10 are plotted, and thus it will be realized that with a value equal to 50, the permissible In the circuit of Fig. 1 the value of g R should be as high as possible, but the values which are conveniently obtainable with ordinary commercial practice are generally satisfactory,

.and permit a firing delay which is of the order of 90 degrees. The air gap in the core of the? output transformer I6 is useful in obtaining the best possible value as it prevents saturation of the core by the direct current flowing through the primary IS.

The closer the firing delay of the tubes in Fig. 1 approaches 90 degrees, the closer the solid wave on axis a in Fig. 4 approaches a pure alternating voltage, while delays of less than 90 degrees introduce a small direct current component into said wave. As will be seen from Figj'l, a full 90 degree delay would require an infinite value of It is therefore necessary to reduce the delay to a value slightly less than 90 degrees, where a more convenient value of wL R If 'the foregoing relation is obtained, the I Fig. 4. The current through the other tube will be identical in shape but displaced by 180 degrees. The sum of these currents will therefore be a steady direct current flowing through the resistance I8 or the resistance of the output transformer primary l5, and producing aloss equal to its product'with the small direct current component of .the solid wave on axis a. of Fig. 4 mentioned above. In actual practice this loss may be made very small.

If an inductive load is connected to the secondary IQ of the output transformer IS, the current through each tube is changed, as shown by the dotted line on axis 1) of Fig. 4, while if a resistance load is connected, the current is shown by the dashed line. A capacitative load produces a modified form of current wave. The maximum 'load which may be applied to the system is that value is equal to the average value of direct current through said primary.

On axis 0 of Fig. 4 is shown a dotted line representing the current in the primary 2 of the input transformer i when the maximum inductive load is applied. The dashed curve represents the same current for the maximum resistance load and the solid wave represents the voltage across the primary 2 of said input transformer I. It will be seen that the dotted line is substantially a pure sine wave having'a zero power factor. The dashed line does not have as good a wave shape but has a power factor of 50 per cent. By means of the condenser 22' connected across the primary 2, this power factor may be corrected to 85 per cent. The ease of correcting the power factor of currents other than sine wave currents depends principally on the degree of harmonics in the wave. Harmonic'components always have zero power factor and cannot be corrected by the simple means such as condensers. If a current is broken up into short pulses, 'it contains harmonics to a greater degree than if the pulses were of longer duration. The operation of the circuit of Fig. 1, whereby the current is maintained through each tube until the other tube starts, therefore reduces the degree of harmonics present in the input current and makes the power factor of '85 per cent. practicable. In actual practice somewhat better values of power factor are obtained due to the fact that small losses in the circuit add unity power factor components to the input current.

The above analysis is made assuming ideal conditions, such as no voltage drop in the tubes.

vAlthough such ideal conditions are not actually analysis. i

In order that the arrangement which I have shown be most efficient, the secondary 3 and the primary i5 should operate at the highest convenient voltages. Inasmuch as the values of these voltages do not depend either on the input voltage or on the load voltage, the specific value of the voltages across the secondary 3 and the primary I5 may be arbitrarily selected. In practice it is easy to obtain an overall efficiency of higher than per cent. with the arrangement 70 as shown in Fig. 1.

If it is desired to convert three-phase power of one frequency to single-phase power of triple frequency, the arrangement as shown-in Fig. 2

may be utilized. In this figure there is il lustrated a circuit which contains an input transformer 22 having three primary windings 23 preferably connected in delta. The transformer 22 is provided with three secondary phases 24, each of which is indicated by a dotted line. It is useful though not absolutely necessary to balance the direct currents through the secondary windings of the transformer 22 in order to eliminate the necessity for providing the core of the transformer 22 with air gaps to prevent direct current saturation. In order to balance the direct currents, each secondary phase 24 is split up into two secondary windings. One set of these secondary windings consists of the coils 25, 26 and 21, 120 degrees apart, and inductively associated with the three respective primary windings. The other set of these secondary Windings consists of the coils 28, 29 and 30 likewise 120 degrees apart and inductively associated with the three respective primary windings. The coils 28, 29 and 30 are wound so as to be 120 degrees away from the first group of windings 25, 26 and 21. The inner ends of the windings 25, 26 and 27 are connected together to form a neutral point 3|. The coils 28, 29 and 30 are connected to the outer ends of the coils 25, 26 and 21, respectively. The position of these various secondary windings in Fig. 2 may represent vectorially the voltages which are generated in said coils, and whenever a winding is shown as parallel to the corresponding winding of the primary 23, it is to be understood that said parallel windings are directly inductively coupled. Thus the three secondary phases 24 appear respectively across the pairs of windings 25 and 28, 26 and 29, 21 and 30.

The outer end of each of the phases 24 is connected by means of a conductor 32 to an anode 33 of a. controlled electrical space discharge tube 34. The tubes 34 are of the same type as tubes 1 described in connection with Fig. 1. Each of the tubes 34 is provided with a cathode 35 and an intermediate electrode 36 which is connected to its cathode by means of its conductor 31. The tubes 34 are each filled-with gas or vapor as previously indicated. In order to control the initiation of a discharge, each tube 34 is provided with a control coil 38 wound upon a magnetic core 39 which is provided with pole pieces which impress a transverse magnetic field across the discharge path within each intermediate electrode 36.

Each cathode 35 is connected by means of a conductor 40 to a common cathode connection 4|. A lead 42 extends from the neutral point 3| through a resistance 43 to one end of the primary 44 of the output transformer 45. The re sistan'ce 43 and the output transformer 45 correspond exactly to the resistance l8 in the output transformer I6 in Fig. 1, and may be constituted in exactly the same way/as described for Fig. 1. The other end of said primary 44 is connected by means of a conductor 46 to the common cathode connection 4|. The output transformer 44 is provided with a secondary which is connected by means of a pair of conductors 48 to a suitable load device 49.

In order to energize the device, the corners of the delta connection of the primary windings 23 are connected by means of conductors 58 to a suitable source of three-phase alternating current. Condensers are connected across each of said three phases in order to correct the power factor as will be indicated below.

In order to cause the control coils 38 to produce the desired time delay in the firing of each of the tubes 34, said coils 38 are each energized from a control device 86 which is preferably a phase-shifting arrangement. One terminal of the input to each control device 86 is connected by means of a conductor 81 to an outer end of the associated phase 24. The other input terminal to each control device 86 is connected to a common conductor 88 which in turn is connected to the conductor 42 extending to the neutral point 3|. By the above arrangement, each control device 86 is energized from the phase impressed upon its associated tube 34.

The behavior in the circuit described in connection with Fig. 2 is illustrated in Fig. 5. The three tubes of Fig. 2 may be represented as tubes A, B and 0. Along the axis a of Fig. 5, the wave shown as a solid line represents the voltage across the primary 44 of the output transformer 45. The dotted lines represent the voltage applied to each anode 33 during the periods when no conduction takes place. As illustrated along axis a of Fig. 4, three alternating voltages, 120 degrees out of phase with each other, are impressed upon the three tubes A, B and C. In absence of the magnetic field due to the coil 38, the tubes A, B and C would normally give the conduction substantially where each voltage wave first intersects an adjacent voltage wave above the horizontal axis. As is well known, this is at an angle of beyond the start of the positive or normally con- 7 may be defined as the normal firing angle. Thus in the present case the time of normal firing thus would be substantially 30 degrees along the voltage wave impressed on said tube. However, the control devices 86 inFig. 2 are set to produce a delay in firing of substantially 90 degrees later than the normal firing time. The system is further designed so that each tube continues to conduct current until the succeeding tube fires. As will be seen from axis a of Fig. 5, such conduction extends through 120 degrees or one-third of a cycle of the power line frequency instead of one-half of such a cycle as in the case of Fig. 1. Since the voltage across the primary winding 44' necessarily follows the anode voltage of whichever tube is conducting, said primary voltage is represented by the solid wave along axis a as pointed out above. It will be seen that this solid wave is three times the frequency of the voltage supplied at the power lines. The secondary winding 41 of Fig. 2 therefore supplies this alternating voltage of triple frequency to the load device 49.

The necessary values of wL R Fig. 1, and therefore operation closer to a 90 degree delay is more easily obtained.

As in the case of Fig. 1, the closer the firing delay of the tube approaches 90 degrees, the

closer the solid wave on axis a of Fig. 5 approaches a pure alternating voltage, while delays of less than 90 degrees introduce a small direct current component into said wave. The curves along axis 2) of Fig. 5 also represent the current flowing through each tube under no load, inductance load, and resistance load, as described in connection with Figs. 1 and 4. Also on axis 0 the solid wave represents the voltage wave of one of the primary phases, while the dotted line represents the current flowing through said primary phase with a maximum resistance load in the load device 49. This current may be corrected by means of the condensers 5| to give a power factor of about 75 per cent.

The two sets of windings constituting each secondary phase 24 in Fig. 2 accomplishes the balancing of direct current as follows. Assuming the phase containing the coils 25 and 28 first' fires, a pulse of current will flow through said windings 25 and 28. However,'120 degrees later, a pulse of current will flow through the coil 29 in the opposite direction to the previous pulse through the coil 25, and thus the direct current in coil 25 is balanced by the direct current in coil 29. 120 degrees later a pulse of current will flow through the coils 21 and 39, and this pulse of current through the coil 21 will balance the pulse of current through the coil 28. In a similar manner the current through the coil 26 balances the current through the coil 30. From the foregoing it will be seen that each primary phase has associated with it two coils which are supplied alternately with pulses of direct current in opposite directions, and thus no resultant D. C. magnetization of any of the primary phases occurs.

One of the principal results to be obtained from the use of the circuit as illustrated in Fig. 2 is that it converts three-phase power to singlephase power of triple frequency. Also the relations of the currents and voltages in all of the three-phase lines are identical. Thus the entire device represents a, balanced load on the three-phase lines, which is a very desirable characteristic.

If it is desired to convert three-phase power of one frequency to three-phase power of triple frequency, the arrangement as shown in Fig. 3 may be utilized. In this figure there is illustrated a circuit having an input transformer 52. The input transformer has three primary windings 53, 54 and 55 preferably connected in delta. The secondary of the transformer 52 is provided with nine secondary phases 55, each indicated by a dotted line. As in the case of Fig. 2 it is desirable that the direct current through the various secondary coils be balanced so that no resultant direct current flux is set up in the core of the transformer 62. For this reason, each of the secondary phases 56 is split up into the various windings, as indicated in Fig. 3. In the interest of clarity each secondary winding in Fig. 3 is placed so as to represent by its length and position, the vectorial value of the voltage which is generated in it. Also when a secondary winding is shown parallel to one of the primary windings 53, 54 and 55, this indicates that it is inductively and directly associated with said primary winding. The secondary windings first of all comprise a group of windings 51, 56, 59, 60, 6| and 62, each of which is wound to produce a voltage equal to 85.5 per cent. of the total desired secondary phase voltage. The coils 5'! and 66 are associated with the primary winding 53, the coils 58 and 6| are associated with the primary winding 54, and the coils 62 and 59 are associated with the primary winding 55.- The inner ends of the coils 51 to 62, inclusive, are connected .at the neutral point 63. An additional set of windings 64 to 69, inclusive, are also associated with the primary windings, and each of these windings 64 to 69, inclusive, are designed to produce 20.1 per cent of each desired secondary phase voltage. The coils 64 and 65 are associated with primary winding 55, the coils 66 and 57 with primary winding 54, and the coils 68 and 69 with primary winding 53. The windings 64 to 69, inclusive, are each connected to the outer ends of the windings 51 to 62, inclusive, as indicated in Fig. 3. The windings 51, 59 and 6| are each provided with a tap 70, H and I2, respectively. These taps are so located as to include between said tap and the neutral point 63, 57.7 per cent. of the total desired secondary phase voltage. Connected to each tap 10, TI and 12, respectively, are three additional windings l3, l4 and 15, each of which likewise is wound to produce a voltage across it equal to 57.7 per cent. of the total desired secondary phase voltage. The coils 13, 14 and 15 are associated respectively with the primary windings 54, 53 and 55. An investigation of Fig. 3 will disclose that the various windings described above are associated so as to produce nine equal phases 56, each displaced from one another by a phase angle of substantially 40 degrees.

A lead 16 extends from the outer end of each secondary phase 56 to an anode ll of a controlled gaseous discharge tube 16. Each controlled gaseous discharge tube 18 has a cathode l9 and an intermediate electrode connected to its respec tive cathode by means of a conductor 8|. Each of the controlled tubes 18 is of substantially the type as described in connection with the controlled tubes 1 of Fig. 1. In order to produce the requisite delay of the firing of each tube 18, each of said tubes is provided with a control coil 82 Wound upon a magnetic core 83. As pointed out previously, such control magnetic cores are constructed so as to impress a transverse magnetic field across the discharge path of each tube 18 within the intermediate electrode 60. The firing delay is of the order of degrees, as discussed in connection with Figs. 1 and 2. Each of the control coils 82 may be energized from its associated phase through a control device exactly as described in connection with Fig. 2. In the interest of simplicity of illustration, such control devices and the circuits thereto are omitted in Fig. 3.

Three of the cathodes l9, displaced degrees from each other, are interconnected by means of a common conductor 89. Three other cathodes 19, also 120 degrees apart and each displaced 40 degrees, from a cathode connected to the conductor 89, are likewise interconnected by means of a common conductor 9'0. The remaining three cathodes 19 are also interconnected by means of a conductor 9|. Three conductors 92 connected respectively to the conductors 89, 99 and 9| are likewise connected to the outer ends of three primary windings 93 of an output transformer 94. Three primary windings 93 are preferably connected in'Y, and are provided with a center or neutral point 95. A conductor 96 is connected to the neutral point 95, and extends through a. re-

sistor 91 to the neutral point 63 of the secondary of transformer 52.

-The three groupings of cathodes 19, as indicated above, separate the controlled tubes 18 into three systems, each identical with that of Fig. 2. However, since the anode voltages 'are displaced 40 degrees in each system from those of the othersythe triple frequency output voltages appearing across the three output transformers on primary windings 93- are displaced 120 degrees from each other, and thus produce a three-phase output. Under these conditions it will be seen that the resistance 91 corresponds exactly to the resistance 43 of Fig. 2. Likewise the criterion of each tube in each of the three systems continuing to conduct current until the succeeding tube of said system starts to conduct current is exactly the same as that given in connection with Fig. 2.

The output transformer 94 is provided with three secondary windings 98 likewise connected in Y. 'The outer ends of the secondary windings voltages, and therefore a delta connection would not be practical.

Inasmuch as Fig. 3 represents three identical systems such as shown in Fig. 2, the analysis as given in Fig. 5 along axes a and b applies with equal force to Fig. 3. Since. each primary phase of Fig.8 has associated with it a number of secondary windings, the summation of the currents as reflected in the primary windings 54 is somewhat different from that indicated for Fig. 2. Fig. 6 represents by the solid wave the primary voltage in one of the phases in Fig. 3, and the dotted line represents the primary current flowing with maximum resistance load. I have found that with such a current,,the power factor may be corrected to better than per cent. by means of condensers. As shown in Fig. 3, the primary windings 53, 54 and 55 are adapted to be connected to a suitable source of three-phase alternating current by means of the conductors I02. In order to perform the powerfactor correction mentioned above, condensers )3 are connected across each of said three primary phases. v

In each of the figures the inductance in the output circuit is represented as being the inductance of the primary winding of the output transformer. Also the load in each case is shown as being coupled to said primary output transformer winding by means of a secondary winding. Instead of using a separate secondary winding, the load could be connected directly across said primary winding. Therefore, when the expression coupled is used to define the relationship between the load and the inductance in the outputci'rcuit of the system, such expression is intended tocover all means of relating the load to said inductance in the output circuit, whether such relation is accomplished by coupling through a separate winding or by connecting the load circuit directly across saidinductance.

Of course it is to be understood that this invention is not limited to the particular details as described above inasmuchas many equivalents will suggest themselvesto'those skilled in the art. For example, thereare many other possible output frequenciesv which may be obtained through various arrangements each incorporating the essential features of my invention. Also in the circuits shown above I have indicated the tubes as individual half wave units, whereas in many cases tubes having a single cathode and a number of anodes, the current to each of which may be controlled, may likewise be used to perform the functions of several tubes in these circuits. Also any type of controlled discharge rectifier in which a firing delay of the type described above may be obtained, may likewise be used. It is therefore desired that the appended claims be given a broad interpretation commensurate with the scope of the invention withinthe art.

What is claimed is:

1. A frequency-converting system comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of degrees beyond the normal firing angle, a common output circuit associated with said rectifiers, means for causing each rectifier to conduct current substantially until another rectifier starts to conduct current, and a higher frequency alternating current load circuit comprised in said output circuit.

2. A frequency-converting system comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, a common output circuit associated with said rectifiers, inductance means in said output circuit for causing each rectifier to conduct current substantially until another rectifier starts to conduct current, and a higher frequency alternating current load circuit comprised in said output circuit.

3; A frequency-converting system comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, a common output circuit comprising effectively a resistance in series with an inductance associated with said rectifiers, said inductance being greater than the critical value at which each rectifier will continue to conduct current substantially until another tube starts to conduct current, and a higher frequency alternating current load circuit coupled to said output circuit.

4. A frequency-converting system comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, a common output circuit comprising a transformer winding, the inductance of said transformer being greater than the critical value at which each rectifier will continue toconduct current substantially'until another tube starts to conduct current, and a high frequency alternating current load circuit coupled to said output circuit.

5. A frequency-converting system comprising a plurality of frequency-changing units each comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of degrees beyond the normal firing angle, and a common output circuit comprising a transformer winding, the inductance of said transformer being greater than the critical value at which each rectifier will continue to conduct current substantially until-another tubestarts to conduct current, said frequency -changing units being vectorially displaced from each other by a predetermined phase angle, said transformer windings being interconnected and likewise being vectorially displaced from each other by a predetermined phase angle,whrereby a plural phase alternating voltage of higher frequency appears across said transformer windin s as a transformer unit, and an alternating current load circuit coupled to said transformer unit.

6. A frequency-converting system comprisin an input transformer being connected to a source of alternating current of a predetermined frequency, said input transformer having a plurality of phase windings each connected at one end to a neutral point, each of said windings being connected at its opposite end to a rectifier so that terminals of one polarity of each of said rectifiers are connected to the outer end of each of said windings, and terminals of theopposite polarity are connected together to said neutral point through a common output circuit, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, means in said output circuit for causing each rectifier to conduct current substantially until another rectifier starts to conduct current, and a higher frequency alternating current load circuit comprised in said output circuit.

'7. A frequency-converting.;-system comprising an input transformer being connected to a source of alternating current of a predetermined frequency, said input transformer having a plurality of phase windings each connected at one end to a neutral point, each of said windings being connected at its opposite end to a rectifier so that terminals of one polarity of each of said rectifiers are connected to the outer end of each of said windings, and terminals of the opposite polarity are connected together to said neutral point through a common output circuit, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, inductance means in said output circuit for causing each rectifier to conduct current substantially until another rectifier starts to conduct current, and a higher frequency alternating current load circuit comprised in said output circuit.

8. A frequency-converting system comprising an input transformer being connected to a source of alternating current of a predetermined frequency, said input transformer having a plurality of phase windings each connected at one end to a neutral point, each of said windings being connected at its opposite end to a rectifier so that terminals of one polarity of each of said rectifiers are connected to the outer end of each of said windings, and terminals of the opposite polarity are connected together to said neutral point through a common output circuit, control means for delaying the starting of current in each of said rectifiers by a phase angle of the order of 90. de-

said core.

on a magnetic core and related to each other to neutralize the direct current magnetization of 9. A frequerfiyonverting system comprising an input transformer being connected to a source of alternating current of a predetermined frequency, said input transformer having a plurality of groups of secondary phases each comprising a plurality of phase windings each connected at one end to a. neutral point, each of said windings being connected at its opposite end to a rectifier so that terminals of one polarity of each of said rectifiers are connected to the outer end of each of said windings and terminals of the opposite polarity are connected together to said neutral point through a common output circuit, control meansfor delaying the starting of current in each of said rectifiers by a phase angle of the order of 90 degrees beyond the normal firing angle, said common output circuit comprising a transformer winding, the inductance of said transformer being greater than the critical value at which each rectifier will continue to conduct current substantially until another tube starts to conduct current, said frequency-changing units being vectorially displaced from each other by a predetermined phase angle, said output transformer windings being interconnected and likewise being vectorially displaced from each other by a predetermined phase angle, whereby a plural phase alternating voltage of higher frequency appears across said output transformer windngs as a transformer unit, and an alternating current load circuit coupled to said transformer unit.

10. A frequency-converting system comprising n rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, each of said rectifiers starting conduction ofcurrent at a phase angle substantially displaced beyond from the start of the normally conducting half of the voltage cycle impressed on the corresponding rectifying phase, a common output circuit associated with said rectifiers, an inductance in series with said rectifiers in said output circuit, and a higher frequency alternating current load circuit coupled across said inductance means.

.11. A frequency-converting system comprising 11 rectifying phases, each supplied with current from an alternating current source of a predetermined frequency, a rectifier in each of said phases, each of said rectifiers starting conduction of current at a phase angle substantially displaced beyond from the start of the normally conducting half of the voltage cycle impressed on the corresponding rectifying phase; means causing each of said rectifiers to continue to conduct current substantially beyond the end' of the normally conducting half of the voltage cycle impressed on said corresponding rectifying phase. and a common higher gflequency output circuit associated with said recers.

12. A frequency-converting system comprising a plurality of rectifying phases, each supplied with current; from an alternating current source of a predetermined frequency, a rectifier in each associated with said rectifiers, an inductance in.

series with said rectifiers in said output circuit, and a higher frequency alternating current load circuit comprised in said output circuit.

13. A frequency-converting system comprising an input transformer connected to a source of alternating current of a predetermined frequency, said input transformer having aplurality of phase windings each connected at one end to a neutral I point, each of said windings being connected at its opposite end to a rectifier, so that terminals of one polarity of each of said rectifiers are connected to the outer end of each of said windings and terminals of the opposite polarity are connected together to said neutral point through a common output circuit, each of said rectifiers starting conduction of current at a phase angle substantialy displaced beyond from the start of the normaly conducting half of the voltage cycle impressed on the corresponding rectifying phase, and a higher frequency alternating current load circuit comprised in said output circuit, said windings being wound on a magnetic core and related to each other to neutralize the direct current magnetization of said core.

14. A frequency-converting system comprising a plurality of frequency-changing units each comprising a plurality of rectifying phases, each supplied with current from an alternating current source of a predetermined frequency; a rectifier in each of said phases, each of said rectifiers starting conduction of current at a phase angle substantially displaced beyond from the start of the normally conducting half of the voltage cycle impressed on the corresponding rectifying phase, and a common output circuit comprising a transformer winding, said frequency-changing units being vectorialy displaced from each other by a predetermined phase angle, said transformer windings being interconnected and likewise being vectorially displaced from each other-by a predetermined phase angle, whereby a plural-phase alternating voltage of higher frequency appears across said transformer windings as a transformer unit, and an alternating current load circuit coupled to said transformer unit.

15. A frequency-converting system comprising an input transformer connected to a source of alternating current of a predetermined frequency, said input transformer having a plurality of groups of secondary phases each comprising a plurality of phase windings each connected at one end to a neutral point, each of said windings being connected at its opposite end to a rectifier, so that termin s of one polarity of each of said rectifiers are connected to the outer end of each of said windings and terminals of the opposite polarity are connected together to said neutral point through a common output circuit, each of said rectifiers starting conduction of current at a phase angle substantially displaced beyond from the start of the normally conducting half of the voltage cycle impressed on the corresponding rectifying phase, and a common output circuit comprising a transformer winding, said frehigher frequency appears across said output.

transformer windings as a transformer unit, and an alternating current load circuit coupled to said transformer unit.

WILQOX P. OVERBECK.

CERTIFICATE OF CORRECTION. I Patent No. 2,185,1 18. January 2, 19110.

WILCOX P. OVERBECK.

It is hereb'v certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 21 -25, for "transvese" read transverse; page 6, second column,

line 71, claim l for "high" read higher; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 50th day of January, A. D. 19110.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

, CERTIFICATE OF CORRECTION. Patent No. 2,185,1 18. January 2, 19110.

WILCOX P. OVERBECK.

It is here'bv certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line Zip-25, for "transvese" read transverse; page 6, secondcolumn,

line 71, claim L for high read higher; and that the said Letters Patent should be read with this correction therein that the same may conform to the record. of the case in the Patent Office.

Signed and sealed this 30th day of January, A. D. 19110.

Henry- Van Arsdale, (Seal) Acting Commissioner of Patents. 

